Chemi-ionization in alkali-methylhalogen collisions

Chemi-ionization has been observed in collisions between the alkali atoms K, Na and Li and the methylhalogens CH₃] and CH₃Br. Total cross sections were measured as a function of the translational energy and the internal energy of methylhalogens. The molecular electron affinities were determined from the thresholds of these reactions. The thresholds indicate that these negative ions are formed by vertical or near-vertical transitions, in contrast to most experiments on chemi-ionization, which yield the adiabatic electron affinities. In the threshold region, the cross sections are strongly dependent upon the internal energy of the molecules. At very high temperature the methyliodide dissociates, and so chemi-ionization was observed with iodine atoms.